How cells build, break down, and speed up reactions
HESI A2 topic: Metabolism
- Anabolic pathways build molecules and use energy; catabolic ones break down and release it.
- ATP is the cell's energy currency; splitting off a phosphate releases energy.
- Enzymes lower activation energy and are not used up.
- Too much heat or the wrong pH can denature an enzyme.
- More substrate beats a competitive inhibitor, not a noncompetitive one.
Key facts
- Anabolic pathways require energy to build complex molecules from simpler ones; catabolic pathways release energy by breaking complex molecules down.
- ATP (adenine + ribose + three phosphates) is the cell's main energy currency; splitting it into ADP and a phosphate releases energy.
- Activation energy is the small energy input every reaction needs to get started, even one that releases energy overall.
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- Enzymes lower activation energy but do not change the total energy a reaction releases or absorbs, and they are not used up.
- The substrate binds the enzyme's active site; in the induced-fit model the enzyme shifts shape slightly as the substrate binds.
- Too much heat or a pH far from the enzyme's optimum can denature it, changing the active site so it stops working.
- A competitive inhibitor blocks the active site and can be overcome by more substrate; a noncompetitive inhibitor binds elsewhere and cannot.
Quick check1 question · 2 more at the end
Which process is an example of a catabolic pathway?
Correct answer: C.
Catabolic pathways break complex molecules into simpler ones and release energy, such as the breakdown of glucose. Anabolic pathways do the opposite: they use energy to build complex molecules from simpler ones. Building proteins, DNA, or sugar are all anabolic.
A Building a protein from amino acids makes a larger molecule and uses energy, so it is anabolic.
B Making sugar from carbon dioxide builds a complex molecule from simple ones, an anabolic example.
C Correct: breaking glucose down into simpler molecules releases energy, which is the definition of catabolism.
D Joining nucleotides into DNA builds a polymer and requires energy, so it is anabolic.
Full lesson
Each part starts with its main idea. Open a part when you want the details.
The short answer: Metabolism is all the chemical reactions in a cell. Anabolic pathways use energy to build complex molecules; catabolic pathways break molecules down and release energy, which cells store in ATP. Enzymes speed reactions by lowering activation energy at their active site, are not used up, and work best at a specific temperature and pH.
1 What is metabolism?Metabolism is the sum of every chemical reaction happening inside a cell. 1 min
Metabolism is the sum of every chemical reaction happening inside a cell. Those reactions are organized into metabolic pathways, chains of steps that turn a starting molecule into a product, with an enzyme running each step.
| Pathway type | What it does | Energy | Example |
|---|---|---|---|
| Anabolic | Builds complex molecules from simple ones | Requires energy | Making sugar from carbon dioxide |
| Catabolic | Breaks complex molecules into simpler ones | Releases energy | Breaking down glucose |
A cell constantly runs both. The energy released by catabolism powers the building done by anabolism.
2 How does ATP carry energy?ATP is the cell's main energy currency, moving energy from the reactions that release it to the jobs that need it. 1 min
ATP is the cell's main energy currency, moving energy from the reactions that release it to the jobs that need it. ATP (adenosine triphosphate) is the base adenine, the sugar ribose, and three phosphate groups in a row.
When the cell splits off the last phosphate (hydrolysis), ATP becomes ADP plus a free phosphate, and energy is released.
Cells couple this release to work that needs energy. For example, the sodium-potassium pump gets its power when a phosphate from ATP is attached to the pump protein (phosphorylation). Rebuilding ATP from ADP and phosphate takes energy, which comes from catabolism such as breaking down glucose.
3 Why do reactions need activation energy?Every reaction needs a small push of energy to get started, called activation energy. 1 min
Every reaction needs a small push of energy to get started, called activation energy. This is true even for reactions that release energy overall.
- Exergonic reactions release energy. They are spontaneous, but spontaneous does not mean fast: rusting iron is spontaneous and slow.
- Endergonic reactions need an energy input to proceed.
Many important reactions in a cell would run far too slowly unless their activation energy is lowered. Lowering it, called catalysis, is the enzyme's job.
4 How does an enzyme work?An enzyme speeds a reaction by giving its reactant, the substrate, a better place to react. 1 min
An enzyme speeds a reaction by giving its reactant, the substrate, a better place to react. The substrate binds a pocket called the active site. Each enzyme is specific: its active site fits particular substrates.
The older lock-and-key idea pictured a rigid, perfect fit. The current induced-fit model says the enzyme changes shape slightly as the substrate binds, which helps push the substrate toward reacting.
Two rules to remember. Enzymes lower the activation energy but do not change how much energy the reaction releases or absorbs. And they come out of the reaction unchanged, so one enzyme can be used again and again.
Many enzymes need helpers: inorganic cofactors such as iron or magnesium ions, or organic coenzymes, often made from vitamins. Almost all enzymes are proteins; see our macromolecules lesson for how protein shape works.
5 What changes how fast an enzyme works?Enzyme speed depends on conditions that affect the shape of the active site. 1 min
Enzyme speed depends on conditions that affect the shape of the active site.
- Temperature: warming speeds reactions up to the enzyme's best (optimum) temperature. Well above it, the enzyme denatures, its active site loses its shape, and activity drops.
- pH: each enzyme works best in a certain pH range. Very acidic or very basic conditions can denature it.
- Substrate amount: adding substrate raises activity until a saturation point, where the enzymes cannot bind any more substrate. Past that, extra substrate does not help.
6 How can an enzyme be blocked?Inhibitors slow enzymes, and where they bind decides how you can overcome them. 1 min
Inhibitors slow enzymes, and where they bind decides how you can overcome them.
| Inhibitor | Binds to | Does adding more substrate help? |
|---|---|---|
| Competitive | The active site, competing with the substrate | Yes, substrate can outcompete it |
| Noncompetitive (allosteric) | Another spot, changing the enzyme's shape | No |
Cells use this for control. In feedback inhibition, the end product of a pathway shuts down an enzyme early in that pathway, so the cell stops making more than it needs.
Common mistakes
The traps students fall into most. Read these before you practice.
Watch out: Anabolic means building (uses energy) and catabolic means breaking down (releases energy), not the other way around.
Watch out: Enzymes lower the activation energy; they do not add energy to the reaction or change how much energy it releases.
Watch out: Raising the temperature speeds an enzyme only up to its optimum; well above that, it denatures and activity falls.
Watch out: Adding more substrate overcomes competitive inhibition, not noncompetitive inhibition.
How to remember it
Memory hook: "Anabolic adds (builds up); catabolic cuts (breaks down)." For inhibitors: a competitive inhibitor competes for the same seat, so flooding the room with substrate wins it back.
Self-check:
- Say whether digesting a protein into amino acids is anabolic or catabolic, and whether it releases or uses energy.
- Explain why an enzyme can be used again and again.
- Predict what happens to an enzyme's activity if you heat it far above its optimum temperature.